In currently widespread nuclear reactor power plants, nuclear fission in a nuclear reactor is accompanied by generation of a considerable amount of radioactive by-products, and since radioactive iodine above all turns into a gas at 184° C., there is a risk that the radioactive iodine is extremely liable to be discharged at the time of inspection or exchange of fuel and furthermore by an unforeseen event such as an accident during handling fuel or a reactor excursion accident. The major radioactive iodine isotopes to be taken into account at the time of discharge are iodine 129 having a long half-life (half-life: 1.57×107 years) and iodine 131 having a short half-life (half-life: 8.05 days). Here, ordinary iodine is an essential trace element in the human body, is collected in the thyroid gland near the throat, and becomes a component of a growth hormone. Therefore, when a human takes in radioactive iodine through breathing or water/foods, the radioactive iodine is collected in the thyroid gland in the same way as in the case of ordinary iodine and increases internal exposure to radioactivity. Accordingly, a particularly strict measure for reducing the amount of radioactivity to be discharged must be implemented with regard to radioactive iodine.
To such a situation, a cleaning processing system, a physical/chemical processing system by a solid adsorbent filling using fibrous activated carbon or the like (see Patent Literatures 1 and 2), a processing by an ion exchange material (see Patent Literature 3), and so on have been studied as a method for processing radioactive iodine generated in a nuclear reactor or the like. And these methods have been utilized in countermeasures against discharge of generated radioactive iodine.
However, any of the above methods have problems as described below, and the development of a method for eliminating radioactive iodine in which these problems are solved is desired. An alkaline cleaning method exists as a cleaning processing system practically used, however there are lots of problems in terms of quantity and safety to carry out processing by the cleaning processing system with a liquid adsorbent and store the processed liquid as it is for a long period of time. In the physical/chemical processing system by a solid adsorbent filling, captured radioactive iodine is always facing the possibility of being replaced with other gases, and in addition to this problem, the processing system has a problem that an adsorbed material is liable to be discharged when the temperature increases. Furthermore, in the processing system by an ion exchange material, the heat resistant temperature of the ion exchange material is up to about 100° C. and there is a problem that the ion exchange material cannot exhibit sufficient performance at a temperature higher than the heat resistant temperature.
Furthermore, in any of the above-described processing methods, large scale facilities such as a circulation pump, a cleaning tank, and furthermore a filling tank containing various adsorbents are necessary, and in addition, there is a practical problem that a large amount of energy is needed to operate these facilities. Moreover, when supply of the power source is suspended as in the accident at the Fukushima No. 1 nuclear power plant in Japan on Mar. 11, 2011, these facilities cannot be operated and the degree of contamination risk by radioactive iodine increases. Especially in this case, eliminating radioactive iodine diffused into peripheral areas falls into an extremely difficult situation, and it is concerned that a situation in which radioactive contamination expands may occur. Accordingly, there is an urgent need to develop a method for eliminating radioactive iodine that is applicable even when the situation in which the supply of the power source is suspended occurs.